Modern wireless communications systems place great demands on the antennas used to transmit and receive signals, especially at cellular wireless base stations. Antennas are required to produce a carefully tailored radiation pattern with a defined beamwidth in azimuth, so that, for example, the wireless cellular coverage area has a controlled overlap with the coverage area of other antennas.
In addition to a defined azimuth beam, such antennas are also required to produce a precisely defined beam pattern in elevation; in fact the elevation beam is generally required to be narrower than the width of the azimuth beam.
It is conventional to construct such antennas as an array of antenna elements so as to form the required beam patterns. Such arrays require a feed network to split signals for transmission into components with the correct phase relationship to drive the antenna elements; when receiving, the feed network doubles as a combiner.
An array consisting of a single vertical column of antenna elements is commonly used as a building block at cellular radio base stations. Such a column antenna can be designed to produce the required narrow elevation beam, and will typically be designed to give azimuth coverage of a sector in a cellular wireless network. In a simple configuration, three such column antennas are deployed at a base station to give coverage to three sectors; this is a form of a spatial division multiple access (SDMA) system, in which the capacity of a cellular wireless system is enhanced by enabling a given frequency band to be used substantially independently by wireless links which are spatially separated.
FIG. 1 illustrates a conventional tri-cellular deployment of base stations. A number of cell sites 1a . . . 1g are deployed to give wireless coverage over a given area. It can be seen that there are three radiation beams roughly equally spaced in azimuth angle at each cell site (for example, in the case of cell site 1a, there are three radiation beams 3a, 3b, 3c). Further capacity increases can be achieved by sub-dividing the azimuth plane more finely in angle, for example to form a hex-sectored plane, as shown in FIG. 2 (in the case of cell site 1a there are six hex-sector radiation beams 5a . . . 5f).
The required azimuth beam patterns for a system such as that illustrated by FIG. 2 can be implemented by the use of multiple column antennas in combination with a beamformer. The beamformer couples together the column antennas in the appropriate amplitude and phase relationship to give the required beam patterns. Such a beamformer will typically be a passive device that may be used for both transmission and reception of signals. Typically, in such a system, the column antennas are referred to as azimuth antenna elements or simply antenna elements.
FIG. 3 illustrates a system in which four azimuth antenna elements 7a . . . 7d are combined to give two beam outputs at ports 11a, 11d for a case in which two beams are required per sector. Each beam may be connected to a respective radio transceiver 27a, 27b, which will typically be connected to a telecommunications network 29 such as the public switched telecommunications network (PSTN). It is known, as shown by FIG. 4, to implement the beamformer 8 by the use of a Butler Matrix 14. The beamformer 8 has 4 antenna element ports 9a . . . 9d, typically connected to an array of antenna elements (as illustrated in FIG. 3 and indicated by reference numerals 7a . . . 7d). It is known to combine pairs of beam ports of the Butler Matrix 20a, 20c and 20b, 20d using 3 dB hybrid couplers 13a, 13b and phase shifters 16a, 16b to produce two beams at beam ports 11a, 11d. However, a beamformer as illustrated by FIGS. 3 and 4 suffers from complexity, involving the use of 6 hybrid couplers and four phase shifters. It should be noted that a beamformer as illustrated by FIGS. 3 and 4 forms beams by combining the contributions from each of the antenna element ports 9a . . . 9d in such a way that signals from an antenna element port cannot be constructively combined at a beam port while signals from another antenna element port are destructively combined at that beam port. Accordingly, it is not possible to access at a beam port the signals received by an antenna element but not those from another antenna element.
It may be convenient to locate the beamformer 8 close to the antenna elements 7a . . . 7d, which will typically be located on an antenna tower. It may also be advantageous in terms of cost, size and performance to integrate the beamformer with the antenna elements, contained within the same enclosure.
However, the integration of a beamformer with its associated antenna elements may present disadvantages in terms of potential upgrade strategies if such strategies require access to the individual antenna elements. In order to access the individual antenna elements, an operator would require to climb the tower and modify or replace the beamformer; in the case of an integrated system this may not be possible, necessitating the replacement of the integrated unit 6. The replacement and re-alignment of antenna elements may be costly; accordingly the lack of an economical upgrade path may limit the deployment of an otherwise attractive integrated beamformer and antenna system.
It is an object of the present invention to provide methods and apparatus which addresses these disadvantages.